Many measuring instruments employ devices for indicating a maximum indication sensed by the instrument in a given period of use. Those instruments having a circular dial on which is marked a graduated scale over which a centrally pivoted needle swings conveniently employ a secondary or auxiliary needle which is engaged by the main needle as it moves in the increasing direction of the scale and released when the needle returns towards its zero position. The auxiliary needle is mounted with a slight frictional resistance to movement so that it remains at the greatest reading reached by the main needle when this latter returns to zero. For instruments having a circular dial with a scale extending less than 360.degree. this is perfectly satisfactory, but for instruments the scale of which extends for more than 360.degree., so that the needle completes more than one revolution upon a full scale deflection, certain problems arise as will be discussed below.
Usually, through the dial of a measuring instrument, extends a spindle which, behind the dial, is mechanically connected to a device sensitive to the physical quantity which the instrument is intended to measure. In front of the dial the spindle carries the main needle which, if the scale on the dial extends for more than 360.degree., is able, as mentioned above, to turn through more than a complete revolution when the physical quantity which the instrument is intended to measure varies by a sufficient amount.
A known kind of maximum reading indicator device includes a transparent screen which can be fitted to the instrument over the dial; through the screen extends a shaft which, when the screen is mounted on the instrument, is aligned with the main spindle which carries the main needle. On the front of the screen the shaft carried a zero setting knob, and behind the screen it carries an auxiliary needle which is rotatably mounted in a frictional manner to this shaft. The auxiliary needle has engagement means by which the main needle causes it to displace when the main needle moves in a sense corresponding to variation of the physical quantity being measured in the increasing sense of the scale.
The present invention will be particularly described herein as applied to a depth gauge of a type suitable to be worn on the wrist by skin divers, frogmen or others who engage in underwater sports. The invention is, however, not limited to this application, but can be employed in relation to any form of measuring instrument having a dial over which a main indicator needle performs more than a complete revolution of the dial upon full scale deflection from zero.
Depth gauges for skin divers and the like must necessarily be small enough to be worn on the wrist. However, in the case of depth gauges for use at great depth, the scale, in order to allow sufficient resolution of the graduations for accurate reading, must have these graduations sufficiently spaced from one another; this means that the scale must inevitably have an angular extent greater than one revolution of the needle.
Clearly, the simple provision of an auxiliary needle for indicating the maximum depth attained cannot be achieved by means of the usual arrangement, described above, in which the auxiliary needle rigidly carries a projection engageable by the main needle because, if the main needle has completed a deflection greater than 360.degree., starting from zero, then upon its return to zero it reaches, after one revolution in the reverse sense from the maximum, the other side of the auxiliary needle and therefore pushes it back to the zero position and the indication of the maximum is lost.
In order to overcome this problem there have been devised maximum indicator devices which, in addition to the auxiliary needle, include a third needle freely rotatable about the same axis as the other two. This third needle has a projection parallel to the axis of rotation which extends into the path of both the main needle and the auxiliary needle. This projection of the third needle is disposed in front of the main needle and behind the auxiliary needle with the increasing sense of the scale being considered as "forward". The main needle, in its forward movement, that is in the increasing sense of the scale, engages the projection of the third needle and the third needle engages the auxiliary needle so that all three move forward together. Upon return of the main needle in the reverse sense, that is the decreasing sense of the scale, from a forward movement of more than one revolution, at the completion of a revolution in the reverse sense from the maximum position reached, the main needle engages the projection of the third needle, on the side opposite to that previously engaged and thereby carries it backwardly, away from the auxiliary needle which thus remains fixed to indicate the maximum reading attained by the instrument. In such known maximum indicator devices, the extra complication due to the presence of three needles is a disadvantage, and likewise the added friction of the two extra needles, that is the auxiliary needle and the third needle, which must be overcome by the main needle upon its forward displacement, is also a disadvantage since, for a precision instrument, it is essential that the friction which the main needle must overcome is minimized in order to obtain the greatest possible accuracy.